Method and solution for phosphatizing metals



Patented Oct. 11, 1960 METHOD AND SOLUTION FOR PHOSPHATIZING METALS No Drawing. Filed Sept. 25, 1958, Ser. No. 763,160 5 Claims. (Cl. 148-615) This invention relates to the phosphate coating art, i.e., to the art of applying to the surface of metals a phosphate coating by means of which the corrosion resistance of the surface is increased and its paint bonding ability is improved. The invention is particularly applicable to the phosphate coating of ferriferous surfaces.

The basic principles of phosphate coating, of course, are now very well known to those skilled in the art so the present disclosure will refer only briefly to the gen eral technique employed.

Probably the most common type of phosphate coating involves what is known as the heavy-metal phosphates generally iron, zinc or manganese, and particularly zinc and manganese. With these phosphates it has long been the practice to employ aqueous acid coating baths or solutions which are saturated or substantially saturated with respect to the metallic phosphate and the coating is produced by applying the solution to the surface either by dipping or by spraying and permitting it to act thereon until a phosphate coating has been produced on the surface of the metal.

However, the basic technique just referred to is open 5 to certain objections among which may be mentioned the fact'that the phosphate coatings are more or less coarsely crystalline in nature with the result that when siccative coatings are subsequently applied thereto the latter often lack resistance to shock or flexion and flake off when the metal is even slightly bent or distorted.

Variousattempts have been 'made' heretofore to overcome this objection such as by adding to the baths suitable so-called accelerators or by employing phosphate solutions which contain no stratifying phosphates or .by a combination of both.

These methods, however, are not always satisfactory from every viewpoint. In recent times, therefore, it has been proposed to use the above-mentioned phosphatizing solutions and suitable additives such as accelerants and anhydrous phosphates, so as to produce a finely crystalline layer instead of the coarse phosphate layer that normally forms. The anhydrous phosphates used in this case, such as the alkali salts of pyrophosphoric, metaphosphoric and polyphosphoric acids, have the disadvantage, however, that they are relatively unstable in the acid, largely hot solutions of the phosphatizing baths, and even in a very brief interval they may be converted in part back to the orthophosphorie acid, resulting in the loss of the refining effect which these substances produce in the layer. Consequently, there is little advantage to adding these compounds to the concentrated initial solutions; instead, they must be added continuously to the diluted, ready-to-use solutions.

The present invention has for its principal object the provision of a method and solution for phosphatizing metals which will overcome the objections above referred to and produce finely crystalline and thin phosphate layers or coatings on the surface of metals especially on the surface of iron or steel, which coatings have excellent corrosion resistant properties and are well suitedas basesfor subsequent application of siccative coatings.

The invention is based upon the discovery that the advantages just'mentioned can be obtained by including in a so-called heavy metal phosphate coating bath, as an addition agent, a relatively small amount of Watersoluble .salt of those phosphoric acids having a P-P bond and in which the oxidation number or valence of a phosphorus atom isless than 5. Furthermore, the invention is useful with phosphatizing solutions which also contain any' of the well known so-called accelerating. agents such as nitrates, nitrites, chlorates or mixtures thereof. Such agents in themselves tend to bring about a certain refinement in the phosphate coating but the present invention still further enhances the results so obtained.

By the oxidation number (of. Holleman-Wiberg, Lehrbuch der anorganischen Chemie, 1955, p. 169, last paragraph) is meant the charge which an atom-a phosphorus atom in the present case-would have if it were composed of ions only. Thus the oxidation number of the phosphorus in anhydrous phosphates such as Na P O or Na P O for example, is 5, whereas it is 4 in the case 'of salts of hypophosphoric acid (H P O Phosphoric acids in which the oxidation number of the phosphorus is less than 5 generally form water-soluble salts with the alkalies, such as sodiurn, potassium, and lithium and with ammonium and magnesium. Watersoluble salts with organic bases, such as triethanolamine or cyclohexylamine, are also known. In this case not all the replaceable hydrogen atoms of the acids have to be replaced by cations or corresponding radicals. The salts may also contain various cations or cationic radicals as for example Na+ and NHJ.

. Typical examples of compounds suitable for the performance of the present process are hypophosphoric acid (H P O and the compounds represented by the following structural formulas: V

In addition, to technically less known compounds of this type, the 'salts *of'hypophosphoric acid, especially, can be used, as for example NaH P O Na H P O Na HP O Na P O or the corresponding potassium,

-lithium,'arnn1onium ;or even magnesium salts; also, the

corresponding salts of hypophosphoric' acid with organic bases such as triethanolamine or cyclohexylamine.

Since hydrolyzation in acid solution takes place considerably more slowly than in the case of the anhydrous phosphates, the above-named salts can be added to the concentrated initial solutions, so that it is unnecessary to be adding them continuously to the ready-to-use solutions: as is the case in practice when hexametaphosphate is used, for example, requiring additional supervision.

It has been found that even with small additions of? water-soluble salts of phosphoric acid in which the oxida-- tion number of the phosphorus is less than 5, the desired effects can be achieved. Such additions are on the order of 0.1 to 10 grams per liter. An addition of 0.2 to 1 gram per liter has proven to be especially effective. Furthermore, the thickness of the layer and the size of thev phosphate crystals can be controlled as desired by the. amount that is added. For example, by addingsomewhat larger amounts, thin, finely crystalline phosphate coatings; are formed which are scarcely visible to the eye.

The water-soluble salts of the phosphoric acids, which are used according to the invention, can be added in the form of a solution, or also in solid form, to the customary phosphatizing solutions. This can be done when the solutions themselves are prepared, or at a later time. Owing to .its relatively great stability in acid solutions, it is unnecessary, as mentioned previously, to withhold the additive until the concentrated solutions are diluted for use. Furthermore, the additives according to the invention can be added, not only to the concentrates, but also to the replenishing solutions customarily used in continuous processes. The phosphatizing solutions according to the invention may furthermore be used both in the dipping and in the spraying method of application.

Typical but non-limiting examples of the invention are given below.

Example 1 A piece of deep-drawing sheet steel was prepared by cleaning it in a known manner with an alkaline cleanser and subsequently etched with 10% sulfuric acid. It was then treated with a hot (65 C.) solution containing 6.75 grams of zinc per liter, 8.75 grams of P per liter in the form of phosphoric acid, and 8.5 grams of N0 After five minutes of treatment the metal was rinsed with water and dried. The surface was found to exhibit a. very thick and coarsely crystalline phosphate coating. Another piece of the same type of steel prepared in the same manner was then treated in a similar solution to which had been added 0.5 gram trisodium hypophosphate per liter. After the five minute treatment the metal was rinsed with water and dried as before whereupon the surface was found to exhibit a very thin and finely crystalline phosphate coating. In the first instance the phosphate layer flaked off partially when the metal was drawn, but no such flaking was observed in the piece treated with the present invention.

Example 2 A phosphatizing solution containing 14.5% zinc oxide, 30% phosphoric acid, nitric acid and 30.5% water was diluted to a concentration of 3% by weight and this solution was heated to 60 C. A piece of deep-drawing sheet steel such as is used in the construction of automobile bodies was cleaned with an emulsion cleaner and was treated by spraying with this solution for two minutes, then rinsed with water and dried. After such treatment a coarsely crystalline phosphate coating had developed on the steel. However, under the same procedure, when alkali hypophosphate was added to the phosphatizing solution in a quantity such that 0.3 gram per liter was contained in the ready-to-use phosphatizing bath a very finely crystalline, firmly adherent phosphate coating was produced.

Example 3 A previously cleaned piece of sheet steel was treated for thirty minutes at a temperature of 95 C. with a phosphatizing solution containing 7.5 grams P 0 per liter in the form of phosphoric acid, and 2.5 grams of manganese per liter. The phosphate layers obtained in this treatment were very thick and coarsely crystalline. However, by the sarne treatment, very finely crystalline phosphate layers firmly adhering to one another are obtained if 0.5 gram of alkali hypophosphate (Na H P O is added to the above-mentioned phosphatizing solution.

Example 4 Previously cleaned pieces of sheet steel were treated by dipping for four minutes in a phosphatizing solution heated to C. and containing 4 grams zinc per liter, 3.5 grams ClO per liter and 12 grams P 0 in the form of phosphoric acid per liter. In this manner, coarsely crystalline, though somewhat finer phosphate layers are obtained on the surface of the sheet steel. If the same procedure is followed, adding 1 gram alkali hypophosphate per liter to the ready-to-use phosphatizing solution, extraordinarily fine, thin phosphate layers are obtained, which are scarcely visible to the eye.

We claim:

1. In the art of phosphatizing metal surfaces the method which includes the step of treating the surface with a solution of heavy-metal phosphate containing, as an addition agent, from 0.1 to 10 grams/liter of a water-soluble salt of phosphoric acid having a P-P bond and in which the oxidation number of a phosphorous atom is less than five.

2. The method of claim 1 wherein the quantity of the salt added is from 0.2 to 1 gram/ liter.

3. The method of claim 1 wherein the treating solution is further characterized by containing accelerating agent selected from the class consisting of chlorates, nitrates and nitrites.

4. A heavy-metal phosphatizing solution containing as an addition agent, from 0.1 to 10 grams/ liter of a watersoluble salt of phosphoric acid having a P--P bond in which the oxidation number of a phosphorous atom is less than five.

5. A solution according to claim 4 wherein the quantity of the salt added is from 0.2 to 1 gram/liter.

6. A solution according to claim 4 containing also accelerating agent selected from the class consisting of chlorates, nitrates and nitrites.

References Cited in the file of this patent UNITED STATES PATENTS 1,485,025 Green Feb. 26, 1924 2,712,511 Orozco et al July 5, 1955 2,846,342 Curtin Aug. 5, 1958 FOREIGN PATENTS 971,926 France Apr. 23, 1950 

4. A HEAVY-METAL PHOSPHATIZING SOLUTION CONTAINING AS AN ADDITION AGENT, FROM 0.1 TO 10 GRAMS/LITER OF A WATERSOLUBLE SALT OF PHOSPHORIC ACID HAVING A P-P BOND IN WHICH THE OXIDATION NUMBER OF A PHOSPHOROUS ATOM IS LESS THAN FIVE. 